The field of the invention is systems and methods for x-ray phase contrast imaging. More particularly, the invention relates to systems and methods for performing x-ray phase contrast imaging using conventional x-ray sources and detectors.
X-rays have been widely used in medical imaging since their discovery in 1895. In medical x-ray imaging, these relatively energetic photons interact with body parts that have different attenuating properties to generate contrast for visualization of the difference between organs. Microscopically, the interaction between x-ray photons and tissue is described by different photon cross-sections due to physical processes such as photoelectric absorption, Thomson scattering, Compton scattering, and pair production. In the diagnostic x-ray energy range (10 keV-150 keV), the pair production can be ignored.
Conventional x-ray systems, such as computed tomography (“CT”) systems, record the attenuation of x-rays due to photoelectric and Compton effects, while regarding the refraction or the angular deviation due to Thomson and Compton scattering as sources of noise. Because the attenuation parameters are very similar in low-atomic-number (“low-Z”) materials, such as soft tissues, these systems cannot adequately differentiate those materials from each other. For instance, absorption-based CT images typically do not effectively distinguish tumors from surrounding healthy tissues.
To overcome this limitation, x-ray phase contrast imaging methods have been developed. When x-rays pass through an object they are not only attenuated, but also experience local phase shifts based on differences in the electron density throughout the object. These local phase shifts can be measured and used as a basis of a contrast mechanism that produces significantly better contrast-to-noise ratio (“CNR”) compared to conventional CT imaging when imaging soft tissues. X-ray phase contrast imaging is thus particularly effective for imaging low-Z materials (e.g., soft tissues or improvised explosives) that have similar attenuation properties.
Although x-ray phase contrast imaging provides significant improvements in the CNR achievable for low-atomic-number materials, its practical implementation has been limited by hardware constraints. Typically, x-ray phase contrast imaging methods require an x-ray source with a high degree of spatial and temporal coherence; namely, a small focal spot size and narrow spectral bandwidth. As a result, x-ray sources used in conventional x-ray imaging systems, such as those used in current clinical practice, are not suitable for phase contrast imaging. Instead, the majority of x-ray phase contrast imaging is performed using an x-ray synchrotron radiation source, which is not practical due to its size and cost, or using a micro-focus source that is too weak to be used for medical imaging applications.
Recently, some methods for performing x-ray phase contrast imaging using a conventional x-ray source have been proposed. For example, in a method called Talbot interferometry, a series of periodic gratings is used, in which one absorption grating is put right in front of the source to increase the spatial coherence of x-rays and two other gratings are put in the imaging chain to record the phase effect. This system requires a careful alignment of the gratings, however, which makes the system not suitable for applications in the field. More importantly, this system is very challenging to scale the size of system up to one that is applicable for scanning a human subject. The difficulty with scaling the Talbot interferometry system is because the thickness of the gratings has to increase as the size of the imaged object increases; however, a thick grating will not allow x-rays to pass through except in the region very close to the center.
In another recently proposed method called spatial harmonic imaging, an image of a two-dimensional periodic structure, such as a wire mesh, is acquired using a high-resolution detector. The local displacement of the image after inserting an object in the imaging chain is connected to the phase shift of x-rays due to the object. This system, however, requires the pixel size of detector to be very small (e.g., on the order of 10 micron), which cannot be adapted in a clinical imaging setting. In addition, the CNR decreases rapidly as the size of the imaged object increases.
In light of these issues, there remains a desire to provide systems and methods for performing x-ray phase contrast imaging at a human scale while using a conventional x-ray source and detector.